Sharks, in one form or another, have been on Earth more than 420 million years. Since they first appeared, they have diversified into about 470 species. They are widespread across the globe, able to adapt to a wide range of environmental conditions and, as an apex predator, seem able to eat just about anything. They are a tough, resilient family of animals.

It would seem that all an aquarium needs to do is plunk a shark down in some salt water, toss in a few feeder fish every day or so and open the doors to the public. Not so easy, according to shark keepers at Adventure Aquarium in Camden, N.J., and the Pittsburgh Zoo & PPG Aquarium.

"The most important thing with keeping sharks is to make sure their well-fed," explained Matt Ferroni, senior biologist at Adventure Aquarium who dives daily in the 550,000-gallon Shark Realm exhibit to check on his charges and their habitat. "Sharks don't wag their tails like a dog does. You can't really tell when they're happy. So we kind of use their health as an indication of what we call happiness.

"They get a variety of food. They're fed frequently. They get their vitamins. Any time we need to do a medical procedure, we have a vet on staff to assist us."Aquarist Ariella Wiener and her fellow shark keeper at the Pittsburgh Zoo and PPG Aquarium, maintain a similarly close watch on the diet on their sharks, which they've trained to come to targets to be fed.

"We can identify all of our sharks. Working with them every day, they all look different. They all have different amounts of food that they get, based on their weight or estimated weight if they haven't been weighed in a bit."

For example, Marge, the largest of the sand tiger sharks at the zoo, is given 1.3 kilograms of fish three times a week, while Wolverine, the smallest of the sand tigers, receives .6-.7 kilograms on the same schedule.

"They have extremely efficient metabolism, so they eat 1-3 percent of their weight three times a week," said Wiener, explaining that the Pittsburgh shark keepers vary what they offer to their sharks among bonita, flounder, mackerel, herring, croaker and bluefish. "They have a wide variety of food items that we give them to make sure they're getting all the nutrients they need.

"They also get vitamins. Sharks have a very specific dietary requirement. So, to make sure they're getting what they need, we give a vitamin supplement specially formulated for sharks and rays." There's a multivitamin aspects to the supplements, but they also are heavy on iron, a nutrient critical to shark health.

Beyond their food requirements, the habitats must also be tuned to the needs of the sharks.

"In (the Shark Realm) exhibit we try to monitor (for conditions similar) to what you're going to find in the ocean, what's natural, "explained Ferroni. "Salinity's at 30-35 parts per thousand. We maintain the temperature at 71-72 degrees."

With an outdoor exhibit exposed to sunlight, the shark keepers at Pittsburgh also fight a daily battle against algae build-up. "We dive in the exhibit every day to help keep it clean, although sometimes it's hard to tell," noted Wiener. "There's so much algae growing in there. It gets a really heavy bioload."